This invention is related generally to a lower tie-plate of a nuclear fuel assembly for a light water reactor, and more particularly to a lower tie-plate with better performance in blocking or suppressing flow of foreign substances entrained by the coolant into the fuel portion, in addition to a method of assembling it.
A conventional fuel assembly for a boiling water reactor has a channel box 1 with a substantially square-shaped cross-section, as shown in FIG. 1. The channel box 1 contains a plurality of fuel rods 2 and at least one water rod 3 arranged in a square lattice. An upper tie-plate 4 and a lower tie-plate 5 are attached to the upper and lower ends of the fuel assembly, respectively. A plurality of spacers 6 are attached to the water rod 3 with specified axial intervals there between, so that the fuel rods 2 may be held in a lattice, although only one of the spacers 6 is shown in FIG. 1.
The lower tie-plate 5 has a network section 7 for holding the fuel rods 2 and the water rod 3 directly, and a nozzle section 8 which extends downward from peripheral portion of the network section 7, so that a lower tie-plate cavity 9 may be formed surrounded by the nozzle section 8 below the network section 7. A lower tie-plate inlet opening 10 is formed at the lower end of the nozzle section 8.
Each of the fuel rods 2 has a cladding tube loaded with a plurality of fuel pellets (not shown), and a lower end plug 11 for closing the lower end of the cladding tube. Lower part of the lower end plug 11 is formed in a slim circular cylindrical rod which is inserted through an insertion-hole 13 formed in the network section 7 of the lower tie-plate, so that the lower end plug 11 of the fuel rod is held there.
The water rod 3 is a hollow metal pipe, and has inlet holes 26 slightly above the lower tie-plate 5 and outlet holes 27 slightly below the upper tie-plate 4 in the channel box 1. The coolant flows into the water rod 3 through the inlet holes 26 in liquid phase, flows upward in the water rod 3 remained in liquid phase, and flows out through the outlet holes 27.
The water rod 3 has a lower end plug 12 at its bottom end. The lower end plug 12 of the water rod 3 is similar to the lower end plug 11 of the fuel rod and has a slim circular cylindrical rod shape which is inserted through an insertion hole 13 formed in the network section 7 of the lower tie-plate, so that the lower end plug 12 of the water rod is supported.
The network section 7 of the lower tie-plate has the insertion holes 13 for receiving and supporting the lower end plugs 11 and 12 as described above, as well as through-holes (not shown) for coolant passes there through between adjacent lower end plugs 11 and 12.
The coolant 15 flows into the lower tie-plate cavity 9 through the lower tie-plated inlet opening 10, passes through the through-holes in the network section 7, flows around the fuel rods 2 and the water rod 3 in the channel box 1, and then, flows out of the fuel assembly through the upper tie-plate 4.
The nozzle section 8 of the lower tie-plate has one or more small leakage holes 17 on its sides, so that a small part of the coolant 15 coming to the lower tie-plate cavity 9 flows out of the channel box 1.
Some fuel assemblies with high performance developed recently have filters for preventing foreign substances from entering the fuel assembly. For example, a lower tie-plate design has a network section with through-holes of about 5 mm in diameters which are smaller than those of conventional designs so that the resistance to the flow or the pressure loss may be increased. Such a design may enhance core stability and also may function as a filter for foreign substances.
The foreign substances which may be expected to enter the fuel assemblies may include small metal wastes remained in the reactor primary containment system during the plant construction, metal brush pieces which have broken off during equipment cleansing and broken pieces which may be results of equipment breaches. The foreign substances may be in various shapes including plates, spiral wires and straight wires.
FIG. 2 shows a prior art lower tie-plate having a filter function for foreign substances (See Japanese Patent Disclosure Hei 7-306284). As shown in FIG. 2, lower parts of the lower end plugs 11 and 12 of the fuel rods 2 and the water rod 3, respectively, penetrate the insertion-holes 13 of the network section 7 of the lower tie-plate 5. A screening plate 20 for filtering foreign substances is disposed below the network section 7 and arranged substantially horizontally across the lower tie-plate cavity 9. The screening plate 20 has many small holes 24 for allowing coolant to flow through while blocking foreign substances, as well as the through-holes 21 and 22 for the lower end plugs of the fuel rods 2 and the water rod 3, respectively, penetrate.
Referring to FIG. 2, the coolant 15 flows into the lower tie-plate cavity 9 through the lower tie-plate inlet opening 10, passes through the small holes 24 in the screening plate 20, and then, passes through the through-holes in the network section 7 into the area around the fuel rods 2 and the water rod 3 within the channel box 1. At this time, most of the foreign substances may be prevented from flowing into the channel box 1, since they would not pass through the small holes 24 in the screening plate 20.
The prior-art lower tie-plate described above could prevent the foreign substances which had reached the inlet area of the core from flowing into the core at a certain probability. However, straight and slim foreign substances might possibly pass through the small holes 24 in the screening plate 20 and the through-holes in the network section 7 when the foreign substances were carried in a position vertically elongated along the flow direction, because the small holes 24 in the screening plate 20 and the through-holes in the network section 7 are aligned substantially linearly upward.